Machines for packaging pourable food products—such as fruit juice, wine, tomato sauce, pasteurized or long-storage (UHT) milk, etc.—are known, on which packages are formed from a continuous tube of packaging material defined by a longitudinally sealed web.
The packaging material has a multilayer structure comprising a layer of paper material covered on both sides with layers of heat-seal material, e.g. polyethylene. And, in the case of aseptic packages for long-storage products, e.g. UHT milk, the packaging material comprises a layer of barrier material defined, for example, by aluminium foil, and which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material eventually defining the inner face of the package and therefore contacting the food product.
To produce aseptic packages, the web of packaging material is unwound off a reel and fed through a sterilizing unit, in which it is sterilized, for example, by immersion in a bath of liquid sterilizing agent, such as a concentrated hydrogen peroxide and water solution.
More specifically, the sterilizing unit comprises a bath filled, in use, with the sterilizing agent, into which the web is fed continuously. The bath conveniently comprises two vertical parallel branches connected at the bottom to define a U-shaped path long enough to ensure the packaging material is treated for a sufficient length of time. For effective treatment in a relatively short time, and therefore to reduce the size of the sterilizing chamber, the sterilizing agent must be maintained at a high temperature, e.g. around 70° C.
The sterilizing unit also comprises a process chamber located over the bath, and in which the web of packaging material is dried; and an aseptic chamber, in which the web is folded and sealed longitudinally to form a tube, which is then filled continuously with the product for packaging.
More specifically, in the process chamber, the web is processed to remove any residual sterilizing agent, the acceptable amount of which in the packaged product is governed by strict standards (the maximum permissible amount being in the region of a few fractions of a part per million).
Such processing normally comprises mechanical removal of any drops on the material, followed by air drying.
The drops may be removed, for example, by feeding the material through a pair of wringing rollers conveniently located close to the process chamber inlet, and downstream from which the material is still covered with a film of sterilizing agent, but has no macroscopic drops.
Drying may be performed by directing jets of sterile air on to the material.
Before leaving the aseptic chamber, the web is folded into a cylinder and sealed longitudinally to form, in known manner, a continuous, longitudinally sealed, vertical tube. In other words, the tube of packaging material forms an extension of the aseptic chamber, and is filled continuously with the pourable food product and then fed to a forming and (transverse) sealing unit for forming the individual packages, and on which the tube is gripped and sealed transversely between pairs of jaws to form aseptic pillow packs.
The pillow packs are separated by cutting the seals between the packs, and are then fed to a final folding station where they are folded mechanically into the finished shape.
Packaging machines of the above type are used widely and satisfactorily in a wide range of food industries for producing aseptic packages from web-fed packaging material. Performance of the sterilizing unit, in particular, ensures ample conformance with standards governing sterility of the packages and the amount of residual sterilizing agent.
A need for further improvement, however, is felt within the industry itself, particularly as regards temperature control of the air used to dry the packaging material web in the sterilizing unit.
Tests have shown, in fact, that, besides drying the web, localized hot-air treatment at the outlet of the sterilizing agent bath synergically improves the effectiveness of the sterilizing agent.
In known machines, the pressure and temperature conditions in the process and aseptic chambers are normally controlled by a closed air processing circuit, which draws air from the process chamber and feeds it back into the aseptic chamber, the temperature of which is controlled by a sensor. The airstream directed on to the packaging material may be generated by “air knives” supplied with air from the sterile chamber, e.g. by means of a recirculating conduit, as described in EP-A-1 050 467.
Since, in this solution, the temperature of the air supplied by the air knives cannot be regulated independently, a process parameter balance designed to simultaneously optimize drying and sterilizing efficiency (“killing rate”) is extremely difficult to achieve.
In an alternative known solution, drying is performed in a low drying channel, through which the material is fed from the process chamber into the aseptic chamber. In this case, too, however, there is no independent adjustment of the air temperature inside the drying channel.
Another problem connected with poor temperature control of the air fed into the aseptic chamber is the risk, in certain operating conditions, of overheating the packaging material, thus resulting in “blistering” between the layers.